1. Field of the Invention
The present invention is directed to a method and apparatus for calculating and monitoring the impedance of an implanted pacemaker lead, and in particular to such a method and apparatus for monitoring the impedance to determine whether a lead fault, such as a lead rupture or a lead dislocation, has occurred.
2. Description of the Prior Art
In general, an implanted cardiac stimulator, such as a pacemaker, includes a pulse generator, and associated electronics, contained in a hermetically sealed housing, and an electrode lead system including one or more electrode cables. Each electrode cable has one or more electrodes disposed at or near a distal end of a sheath of insulating material surrounding an electrical conductor, and has a proximal end with an electrical connector which is connectable to the pulse generator housing at a connector jack located on the housing. Typically, each electrode cable is implanted by advancing the electrode cable through a vein and into the atrium or ventricle of the heart which is to be stimulated. Usually, it is desirable that the electrode or electrodes carried at the distal end of the cable be located in the heart at a relatively specific position, in order to achieve a desired stimulation therapy. The pulse generator housing is implanted at a location remote from the distal end or ends of the electrode cable or cables, typically at a sub-clavial or an abdominal location.
In order for the desired pacing therapy to be administered by the pacemaker, it is important that a complete (closed) electrical circuit be present, including the pulse generator and the electrode cable or cables attached thereto. It is common to employ the pacemaker housing as an indifferent electrode, so that a return path for current, following delivery of a stimulation pulse, exists between the stimulation electrode and the housing through body tissue. Interruptions or modifications of this complete circuit can occur, for example, if the electrical connection at the housing jack becomes contaminated with body fluid or otherwise becomes loosened, or if a rupture occurs at some point along the length of an electrode cable, or if the tip electrode becomes dislodged from its intended placement position in the heart. If any of these malfunctions occurs, it is important to be able to detect the occurrence of the malfunction as soon as possible.
All of the above types of malfunctions will result in a noticeable change in the impedance of the lead system, causing the impedance to deviate noticeably from a "normal" or base value. In order to detect the occurrence of such malfunctions, therefore, it is well-known to include impedance monitoring circuitry in an implantable pacemaker, so as to be able to check periodically to determine whether the lead impedance deviates significantly from the nominal value. If a significant impedance deviation is detected, it is assumed that a malfunction in the lead system has occurred, and some type of warning indicator can be generated.
Since the lead impedance may vary due to a number of different factors, other than the presence of a malfunction, the aforementioned nominal impedance must be associated with a known amplitude, duration and rate of the pacing pulses. In order to detect deviations from this nominal impedance, the pacemaker must at the time of the measurement deliver pacing pulses having the known amplitude, duration and rate. In many instances, however, the amplitude, duration and rate of the pacing pulses which are associated with the nominal impedance may not be the most effective combination for treating the particular cardiac pathology of a person in whom the pacemaker system is implanted. Therefore, if the pacemaker, for administering stimulation therapy, is operating so as to deliver pacing pulses which are different from the pulses associated with the nominal impedance, the pacemaker must be forced to operate temporarily in a mode in which it emits pacing pulses having the amplitude, duration and rate associated with the nominal impedance. During this mode, the aforementioned impedance measurements are made to determine whether deviations from the nominal impedance exist.
In modern pacemakers which are controlled by telemetry by means of an external programmer, a physician or the patient can operate the programmer in order to set the pacemaker to the aforementioned operating mode to permit the impedance measurement to be made. This has the disadvantage, however, of temporarily administering pacing pulses which are different from the pacing pulses that are most effective for treating the patient's particular cardiac pathology, and also requires that the pacemaker, upon completion of the impedance measuring mode, must be reprogrammed to return it to its previous operating parameters.
Examples of implantable cardiac systems which include impedance monitoring of the type described above are disclosed in U.S. Pat. Nos. 4,140,131; 5,423,871 and 5,431,692, and in European Application 0 360 551.
It is also known to identify the presence of a lead malfunction by obtaining a surface electrocardiogram (ECG) of the patient. The "normal" waveform of the ECG is known, and the presence of malfunctions of the type described above will cause artifacts to appear in the waveform which can be recognized on the ECG monitor. For ECG monitoring of lead faults, therefore, it is the ECG waveform itself which is viewed and analyzed, rather than using impedance as the fault-identifying parameter.